Warning: Title::newFromTitle: $text must be a string:
from
HTMLForm::tryAuthorizedSubmit/HTMLForm::trySubmit/SpecialGlobalRenameProgress::onSubmit/User::getCanonicalName/Title::newFromText
function onSubmit( array $data ) { $name = User::getCanonicalName( $data['username'], 'usable' ); //\-> Title::newFromText
Looks like username can be unset (and yield null), or otherwise contain a non-string value.